The F.D.A. suspends use of a Glaxo antibody drug in the U.S. as an Omicron subvariant spreads.

Federal regulators said they were suspending use of a monoclonal antibody drug known as sotrovimab to treat high-risk Covid-19 patients in the United States because it was unlikely to be effective against the Omicron subvariant known as BA.2.

BA.2 is highly transmissible and is now dominant in the United States, four months after it was first detected in the country. The Centers for Disease Control and Prevention estimated on Tuesday that BA.2 accounted for about 72 percent of new U.S. coronavirus cases in the week ended April 2.

In late March, federal health officials stopped shipments of sotrovimab to eight states in the Northeast and two territories — Puerto Rico and the U.S. Virgin Islands — where BA.2 had become dominant earlier than in the rest of the country.

Doctors treating high-risk patients can still use another monoclonal antibody drug, bebtelovimab, which is manufactured by Eli Lilly. There are also three antiviral treatments that have been found in laboratory tests to be potent against BA.2.

Sotrovimab was widely used during the surge in cases over the winter because it was the only authorized antibody treatment that worked against BA.1, the Omicron subvariant that was dominant at that time.

In January, federal officials restricted use of two other antibody treatments, from Eli Lilly and Regeneron, that were widely used during the Delta surge last year because they were not potent against BA.1. They are not expected to work against BA.2 either.

Sotrovimab’s manufacturers, GlaxoSmithKline and Vir Biotechnology, said in March that they had found in laboratory testing that the authorized dosage of the drug was not sufficiently potent against BA.2, matching earlier findings from independent researchers. The companies said they were preparing to submit data to regulators to seek authorization for a higher dose that they hope will work against BA.2.

Rebecca Robbins contributed reporting.